William D. Thornbury
Principles of Geomerphology
Concept 1
The same physical processes and laws that operate today operated throughout geologic time, althought not necessarilly always with the same intensity as now.
This is the underlying principle of modern geology and is known as the principle of uniformitarianism. It was first enunciated by Hutton in 1785, beautifully restated by playfair in 1802, and popularized by Lyell in the numerous editions of his principles of Geology. Hutton taught that "the present is the key to the past," but he applied this principle somewhat too rigidly and argued that geologic processes operated throughout geologic time with the same intensity as now. We know now that this is not true. Glaciers were much more significant during the Pleistocene and during other periods of geologic time than now, world climates have not always been distributed as they now are, and, thus, regions that are now humid have been desert and areas now desert have been humid, periods of crustal instability seem to have separated periods of relative crustal stability, although there are some who doubt this, and there weretimes when vulcanism was more important than now.
